The present invention relates to a method and apparatus for disassembly of sheet metal structures which have been spot welded together, and to a novel hand tool for performing such method in the auto body industry.
Spot welding is a widely used assembly technique for all manner of structures in which different sheets of metal must be fastened to each other. Examples of such structures are appliance cabinets, chassis for electronic parts, airplanes and perhaps most ubiquitously, automobile bodies. Spot welding is not only inexpensive and fast, but has the esthetic advantage of not involving fastener heads or other elements protruding above the surface of the sheet metal being fastened. In an automobile, numerous curved, stamped sheets of metal, such as those comprising fenders, rocker panels, reinforcements for the perimeter of the trunk, and other such pieces are spot welded into the final assembly at the factory. When an automobile is involved in a collision, or because of rust or deterioration requires replacement of such assemblies, the spot welded sheet must be separated from the undamaged portions of the vehicle and a new contoured sheet assembly must be installed. However, because the damaged assembly is spot welded, certain problems arise in disassembling the structure. With a simple bolted structure, it is common to burn off the bolt heads with an acetylene torch or to unscrew the bolts, allowing the sheets to simply separate. However, with the spot welded structure, the prevalent methods of disassembly are either tedious, or require force which may be destructive of other portions of the car.
One existing method is to painstakingly drill out each spot weld. Drilling out spot welds is accomplished by drilling first a small pilot hole through the middle of each spot weld and then using a circular piloted bit to entirely remove the weld from the exterior sheet. This operation must be done for each spot weld, to separate the portions of metal fused to the exterior sheet, after which the two sheets will simply separate. The protruding welds remaining on the lower sheet must then be ground flat. This process of removing welds, while resulting in a neat work product, is extremely tedious.
A second method of disassembling spot welded structures is to separate the two sheets at some point, and then to drive a dull chisel with a sharpened notch along the seam between the sheets so as to pry the sheets apart while severing the fused portions. Such a chisel, the use of which is now common together with a pneumatic handle assembly in auto body shops, is shown in U.S. Pat. No. 3,191,909 issued June 29, 1965 to N. M. Reischl. The use of a pneumatic chisel or parting tool in this manner, while effective, entails the use of force which may bend, rip or alter the underlying contour of the otherwise undamaged metal pieces, especially when used on the thinner gauge steels now common in the auto industry. Furthermore, such a parting tool and pneumatic hammer together form a rather bulky assembly, and may be serviceable only in situations where the spot welded seam is very exposed, without other structures or contours of the workpiece obstructing the area of work for a large distance on all sides.
A second specialized tool for disassembly of sheet metal structures operates with a rotary driver and essentially works in the manner of a sardine can opener. Such a device is shown in U.S. Pat. No. 3,688,383 issued Sept. 5, 1972 to A. M. Martin. It is not known whether the tool shown in that patent has been made or marketed; however that tool also involves parting the welds by force, essentially pulling them apart along a narrow band immediately surrounding the line of spot welds. In addition to exerting tearing force along the spot welds, that tool requires substantial clearance for insertion of the tool along the length of the seam, and also would appear to work only for the removal of a well exposed piece from another, substantially thicker, piece. As such, there are numerous portions of an automobile body, such as curved pieces, or pieces having vertical corners rising quite near to the spot weld, or pieces having spot welds situated at the base of a shallow channel or trench, as in the perimeter of a trunk compartment of an automobile, where none of the above specialized tools would work, and where the laborious drilling out of individual spot welds by hand would be the only practicable method, if any, of disassembly or of nondestructive disassembly. Finally a clamp-like structure having a pneumatically driven end mill has recently appeared on the market for removing spot welds. This device has the end mill and drive motor affixed to one arm of a C-shaped clamp, so as to be moved down toward the opposing arm of the clamp. As the clamp is closed, the mill cuts through one of a pair of spot welded sheets. That device is a logical extension of the usual art of drilling out welds, but is bulky, requires adjustment to suit the gauge of metal involved, is subject to bit wear, and may "skitter" on curved seams, or if incorrectly aligned.